Eye Movements as a Test Case for the Equilibrium Point Hypothesis

James R. Tresilian

Department of Human Movement Studies, University of Queensland,
Brisbane, Australia

The equilibrium point (EP) hypothesis in
the form proposed by A. G. Feldman emphasizes the role of stretch reflex mechanisms
in motor control. In Feldman’s formulation, the neuromuscular control variable
can be identified as the threshold of the tonic stretch reflex and is denoted
by the symbol "l"
(lambda). Eye movements are potentially an important test case because the eye
movement system has is unique in ways that present a significant challenge for
the EP hypothesis. First, it is widely accepted that the extraocular muscles
are unique in that they lack any stretch reflexes and so they may violate the
basic tenet of the EP hypothesis. Second, saccadic eye movements involve some
of the fastest muscle contractions in the human body for which the asymmetric,
non-linear viscosity of active muscle (usually neglected in EP models) will
have a significant effect on movement kinematics. In addition, reflex time delays
(if present for eye movements) would also be expected to have significant effects
on kinematics. Finally, unlike other systems, the form of the central commands
to ocular motor neurons is well documented and appears to contradict the form
of these commands suggested by Feldman. A model of the horizontal eye movement
system is presented based on the EP hypothesis and its success in accounting
for available data is considered. It is concluded that the EP hypothesis can
provide the basis for a realistic model of eye movement control.